1. Field of the Invention
The present invention relates to a method for producing nanostructures and nanostructures obtained by the same method. More particularly, the present invention relates to a method for producing nanostructures having a desired shape easily with excellent cost-efficiency and safety by allowing nanostructures to grow in a liquid phase environment, unlike a conventional vapor-liquid-solid (VLS) process performed in a vapor phase, as well as to nanostructures obtained by the same method.
2. Description of Related Art
NEMS is the abbreviation of nanoelectromechanical systems and refers to the field of nano-scaled ultra-compact precision machines smaller than microelectromechanical systems (MEMS). One of the outcomes of such NEMS is to produce sensors and actuators based on nanowires.
FIG. 1 is a schematic view showing a process including formation of nanostructures, growth of the nanostructures, recovery of the nanostructures and assembly of the nanostructures in accordance with the prior art.
As shown in FIG. 1, the process for applying nanowires to a device according to the prior art includes: growing nanowires on a separate substrate by a catalyst; separating and recovering the grown nanowires from the substrate; and assembling and aligning the nanowires in a device, such as an NEMS device. However, the above process is not cost-effective in that the nanowires have to be separated from the substrate through an additional step after they are grown, and then assembled and aligned in a device.
In addition, the most general process of the conventional nanowire growing processes is a vapor-liquid-solid (VLS) process. The VLS process was suggested by Wagner et al. in 1960's in order to grow micrometer-scaled single crystals. Recently, the VLS process has been applied to the growth of single crystalline nanowire structures of inorganic compounds by many researchers.
FIG. 2 is a schematic view showing a VLS mechanism for the growth of nanowires.
As shown in FIG. 2, the solid/liquid phase equilibrium diagram of a mixture demonstrates that an alloy of two metal substances melts at a temperature lower than the unique melting points of the two substances. In the VLS process, nanoparticles of noble metals, such as gold (Au) or silver (Ag) which are stable at high temperature and have a relatively lower melting point, are applied to a substrate, and a substance to be grown or a precursor thereof having a relatively higher melting point is allowed to evaporate at high temperature. As the evaporated reactant gas dissolves into the liquid droplets of the noble metals, the reactants reach a supersaturated state. While the supersaturated reactants pass through a liquid state and are precipitated in the form of a solid material, the solid material grows in one direction, and thus a one-dimensional substance grows. In general, metals are used as the growing substance to maintain the phase equilibrium with ease, and the atmospheric gas is controlled for a metal compound structure, such as a compound semiconductor, to control the form of oxides or sulfides.
The reaction procedure of the VLS process starts with dissolution of a reactant gas in a vapor phase into nano-sized molten particle droplets functioning as a catalyst. Herein, as the concentration of the reactant gas increases, nucleation of the reactant is performed by the phase equilibrium of the mixture. Then, such nucleation is used to grow single crystalline nanowires or nanorods. Ideally, one-dimensional growth is maintained by the molten metal nanodrops and the grown nanowires have a diameter or thickness determined by the size of the metal nanodrops.
However, the conventional VLS process has the problems as described hereinafter.
First, high temperature is required to perform evaporation of a substance to be grown or a precursor thereof. In other words, since the gas of the precursor, etc. should be dissolved into a liquid catalyst at a temperature above the eutectic point of alloy, a relatively high temperature is required.
Next, in addition to such a high temperature, a harmful or flammable gas is used essentially. Thus, the work environment is very harmful and dangerous. For example, SiH4 that may explode should be used in the case of silicon nanowires (SiNW), GeH4 that are toxic and may explode should be used in the case of germanium nanowires (GeNW), and methane that may explode should be used in the case of carbon nanotubes. Under these circumstances, it is very difficult to control or regulate the work environment. In some cases, the operators may be exposed to a hazardous environment.
In addition, even though nanostructures are obtained through the VLS process, the resultant nanostructure should be integrated and assembled again to obtain a desired shape. Thus, the overall process is complicated and is not cost-effective. Moreover, when the resultant nanostructures are integrated or assembled in an additional step, it is practically difficult to produce nanostructures with various shapes at a desired position.
Further, according to the prior art, there has been an attempt to carry out the VLS process using a heater or laser so as to integrate the three steps, i.e., growth-separation-assembly, into one step. However, such an attempt is still problematic because of the fundamental limitation of the VLS process itself, low flexibility in an operational process change, and a need of an expensive vacuum chamber due to the use of gas precursors. Moreover, there is a technical limitation in producing various types of nanostructures except carbon nanotubes or single crystalline nanowires.